Use the encounter building guidelines in the Dungeon Master's Guide
Encounter building tools can be found in the Dungeon Master's Guide, page 81 on. They can also be found in DnD Beyond (which can also do the math for you).
For example, an encounter against ten homunculi would have an adjusted XP value of (2.5 × 10 × 10XP) = 250XP, where 2.5 is the multiplier for 7-10 hostiles, 10 is the number of the homunculi, and 10XP is the experience value of a single homunculus. For four level 1 PCs, this encounter would qualify as Medium.
To get the XP value for monsters in an encounter, you can use the CR to XP table on Monster Manual page 9. Each monster's XP value is also included in its stat block. CR 0 monsters are worth 0 or 10 XP depending on whether they have a proper attack or not.
The slightly modified encounter building scheme of Xanathar's Guide to Everything (also available in DnD Beyond) presents a simpler, not quite equivalent form:
For low challenge ratings not appearing on the table, assume a 1:12 ratio, indicating that twelve creatures of those challenge ratings are equivalent to one character of a specific level.
Remember that while the encounter difficulty calculation is a useful tool in normal situations, it's always the best to weigh the output against the strengths and weaknesses of the particular party you are placing the encounter against.
Following the monster creation guidelines in the DMG, you can quickly bump the HP and damage of your existing Elementals to make more powerful ones that should fall within the guidelines.
Every row in the table above Challenge 1 applies a pretty consistent difference in HP and damage to the creature, which means it doesn't really matter what kind of compensation the base creature has, or what the normal difference between its Offensive and Defensive CR is, because you're just sliding it a few rows up both sides of the table and the new creature should have roughly the right kind of Challenge.
This should work for essentially any creature, although the results are likely to be a bit wonky with lower level ones. Fortunately, Elementals start at Challenge 5 and at that point it all works out pretty well.
So, more specifically, each increase in Challenge adds to the creature an extra 15 HP, and increases its damage by about 6 per turn. Attack bonus, save DC and AC go up by one roughly every 4 levels.
So to rapidly bump an Earth Elemental from Challenge 5 to Challenge 8, you could just:
- Increase its HP by 15 x (8-5) = 45
- Increase its damage by 6 x (8-5) = 18 per round
- Increase its AC, attack bonus and Save DC by 1
The easiest way to raise the Earth Elementals damage by 18 is by giving it a third Slam on its Multiattack and bumping the damage for each by 1, since each deals about 14 points of damage with each. You could also bump the existing ones by 9 damage each, by throwing in an extra 2d8 damage on them.
Following those steps shouldn't take more than a minute or so per creature, and gets you something with a Challenge roughly as reliable as the original creature.
Best Answer
When the Monster Manual's Manticore (page 213) uses its Multiattack, it can either combine a bite and two claw attacks, or it can make three tail spike attacks. A bite and two claws comes out to 19 damage, on average, and three tail spikes deal a combined 21 damage, on average. The DMG tells us to make CR judgments based on the more damaging option available to a monster, so the Manticore gets to use 21, but our spikeless Dragonne is limited to 19.
Looking at the Monster Statistics by Challenge Rating table (DMG 274), 21 damage per round is in the range of a CR 3 monster. 19 damage per round is in the range of a CR 2 monster. The cutoff is 20/21, so this is a fairly fine distinction we're making.
However, as described under "Final Challenge Rating" (on the same page), monster CR depends on other factors. The Manticore's HP falls in the CR 1/2 range, and its AC is in the CR 4 range, so its Defensive Challenge Rating comes in at 2. Our Dragonne's new damage per round is in CR 2 range, and its attack bonus is in the CR 4 range, so its Offensive challenge Rating is 3.
We get the final average rating by calculating the average of the monster's offensive and defensive challenge ratings: 2.5. "Round the average up or down to the nearest challenge rating," the DMG says. According to the text, we're free to treat the Dragonne as a CR 2 or a CR 3 monster.
But all this math only confirms what was pretty obvious in the first paragraph: Numbers-wise, we haven't changed the Manticore very much. We've decreased its average damage per round by 2, which in many cases wouldn't be enough to affect the CR at all.
Removing the Manticore's tail spikes may be an important change strategically: The Manticore is (roughly) as dangerous at range as it is in melee, but our Dragonne doesn't have a ranged attack. The DMG's guidelines for monster creation and CR calculation don't say anything about the difference between ranged and melee attacks, though, which means they don't affect Challenge Rating, the abstract, mathematically-defined concept, at all. Whether the Dragonne's reliance on melee has a calculable effect on how challenging it is in "real life" will come down to how you use it.